Assigning elevation data to image?

PhilRatcliffePhilRatcliffe Global Mapper UserPosts: 27Trusted User
edited January 2015 in Raster Data
Hi, I have an image file & have numerous spot elevations that I have surveyed myself within the image bounds. I recall reading somewhere that known elevation points can be used with an image to generate elevation data throughout the image for the areas where no survey has been carried out. Is this possible or was I imagining it??
thanks Phil


  • PhilRatcliffePhilRatcliffe Global Mapper User Posts: 27Trusted User
    edited January 2015
    Have i asked a stupid question? I have searched the forum with no luck, unless I'm searching with the wrong key words??
  • MykleMykle Global Mapper User Posts: 452Trusted User
    edited January 2015
    Your image is probably independent of your data points.

    If you have coordinates with elevations for your spot points, then you can import them and use
    | Analysis | Create Elevation Grid
    (you didn't provide the version that you are running, so this is a v16 selection. older versions will have a different selection to get to the same results.)

    If you have elevations but not coordinates, and you can identify the location of each point on the image, then you can create points using the image as a guide and assign an elevation to each point. You can then turn off your image as it is no longer needed to create your surface. Export your points and elevations to a data file, then reload it.

    Once your elevation grid is generated, you can use hillshading to look at the grid. You should export the elevation grid (use the Global Mapper Grid format unless you have specific needs) to a separate file, then reload it.

    You can also download other elevation data, like SRTM and ASTER (almost worldwide) or DEM (USA, Canada, Mexico). These may be more distracting rather than useful depending upon your needs.

    Order your loaded layers with the elevation grid first (at the top in the Overlay Control Center), then your image and any other layers you may have, like a topo map. You will be able to use the 3D View to look at the image draped over the elevation grid.

    You may have noticed recommendations to export data that you generate within the program. While you can save your work in a workspace file (recommended), your data will be embedded. If you export your data (points, elevation grid, etc) to separate files and load them separately (and delete the original layers when you are satisfied that the data files are complete), then your workspace file will be smaller.

    Depending upon your layers, the workspace may open (much) more quickly from separate files. While your layers appear to be relatively simple, some layers can become complex. For example, topo maps may be provided as Acrobat PDF files that Global Mapper will rasterize. The map can be exported to a geoTIFF, which will load relatively quickly. Otherwise the program will need to re-rasterize when you open your project. Layers of data downloaded from the internet are another example. If you export the data to a local file, the program will not need to re-download the data.

    Your question was not stupid (none are!). It is simply a broad question. Suggest that you always include the version number of your program, as it is almost always the first reply that you will get. In recent versions, use
    | Help | Global Mapper Forum
    to copy the details that can be pasted into your message.
    Here's mine: Global Mapper v16.0.5 (b111714) [64-bit] - REGISTERED

  • PhilRatcliffePhilRatcliffe Global Mapper User Posts: 27Trusted User
    edited January 2015
    Thanks Mykle, the question wasn't meant to be that broad. What I am asking is if I know the elevation at a certain point (Vector) & I have an image (Raster) can I use that point to calculate an elevation profile (contours) for the entire image bounds. I assume it would mean converting the image into a 3D image?? I hope this makes it a little clearer.
    I am using Global Mapper v15.2.9 (b090414) [64-bit] - REGISTERED
    Thanks Phil
  • MykleMykle Global Mapper User Posts: 452Trusted User
    edited January 2015

    Thanks for the additional description (and version).

    A raster image does not contain any elevation data.

    A 3D image is probably going to be a product of several layers, like elevation and image. Some of the 3D formats that GM can use may provide combined data (another discussion).

    Note that elevation data CAN be provided as a raster file, and GM can be told to import the file as elevations rather than as an image. Your file appears to be an image, so again there is no elevation data included.

    So you need to acquire elevation data from elsewhere, whether as a download (SRTM, ASTER, DEM, LIDAR, etc), as a file that you provide (same types of data), or as points that you enter within the program.
    | File | Download Online Imagery/Topo/Terrain Maps
    The TERRAIN DATA section includes servers that provide a variety of data at different resolutions. LIDAR data are in a different section.

    Elevation data from one source is best. Combining data from different sources (including manual input) can be problematic.

    I'll suggest that you set aside your independent elevation data for now. Download data from an on-line source to see what you get for your area. SRTM is likely to be at the lowest end of available data (around 80m resolution). ASTER will be a bit better (around 50m). DEM data covering the USA is available at 30m and 10m resolution. Mexico may need to be downloaded separately from USGS (30m). I'm less familiar with obtaining data for Canada. And dunno about other countries. You can download from each source and export to an elevation file, then load all of the elevation files and toggle them as you like for comparison.

    LIDAR data are available for some areas at higher resolution. That is worth a separate discussion, if appropriate.

    Again, when you download data (or convert data to a different format), I will export that data to a separate file (then remove the first layer after loading and verifying the new file). Otherwise you have to download it again each time you open your project, and that assumes that your source remains available. It is much faster and more reliable to have local files. Note that the export dialogs have an Export Bounds tab that likely defaults to "All Loaded Data". You probably will want to use "All Data Visible On Screen", after you have ensured that all of your area of interest is included. This setting is NOT sticky, so you have to remember to select the setting for each export. There IS an option somewhere that changes the default, but I'll have to dig it out. (there are several Configuration options that may be relevant, but the appropriate one didn't stand out on a quick review).

    When you export downloaded data, note the values for Sample Spacing. If you are saving data covering a large area, you may need to increase the sample spacing. The export will use the maximum available resolution by default, and that may require an extended period of time to complete (if at all). I suspect that you are working with a relatively small area (few square miles or kilometers), so this may be a minor concern.

  • PhilRatcliffePhilRatcliffe Global Mapper User Posts: 27Trusted User
    edited January 2015
    Thanks again Mykle, herein lies the issue, I'm in Australia & unlike the US we have very little in the way of DEM's of the areas I'm interested in and what little LiDAR Data that has been acquired is not available to the public (even though our taxpayer dollars funded it). I have been working on this for over a year & have become very frustrated with the lack of available data, to the point where I have almost given up.
    I searched through & found the article I read back in April & this is the process I am attempting to "define".
    "Airborne LiDAR is, as best as I know, is not flown on regular areas or blocks in Australia. In general LiDAR is flown by private contractors (AAM, Fuguro, etc) who sell the data to whomever. So getting one’s hands on the data is not easy and usually costs dollars. But, we often see aerial photographs flown on a regular basis, again by private contractors and often for Government. The quality of aerial photography these days is very good and one can generate 3D data largely automatically. The 3D data can be in contours or in 3d grids or as point clouds (with the same format as LiDAR).

    Finding the highest water level is usually easy with photogrammetry as images of the spillway provide the location of the highest water level.

    Of course a more approximate way to do this is to use the shape of the surrounding topography to interpret the shape of the terrain without a dam. We have used tensioned beta splines for this. Then add back the location and height of the dam wall (guessing the shape is symmetrical from the bit of the wall that can be seen) and finally the location and height of the spillway to get the maximum surface height. Then used calculations to obtain volume."
    Thanks again for your time in answering my question.
  • MykleMykle Global Mapper User Posts: 452Trusted User
    edited January 2015
    Hi Phil,

    Ah, Australia. I thought it was all flat !

    Yes, I understand about the lack of DEM and LIDAR, and the prices charged. Not fun.

    Generating elevations from Photogrammetry is a major project that assumes the aerial survey has covered the area with over 50% overlap between flight lines. It takes two photographs to generate vertical data, assuming you have all of the parameters.

    Global Mapper is not presently able to import raw aerial photographs and convert them for use. Books are available on the subject, and the software is expensive.

    Assuming that SRTM or ASTER data don't provide a usable amount of detail, you are left with importing a topo map and digitizing points along contours. There are some programs that can generate contours from a raster map, and that are inexpensive if not free (it has been awhile since I did a project using them). The results need to be cleaned up by hand, but it is better than doing it all manually. That assumes that you have a pretty clean topo map without a lot of layers that confuse an automated procedure, and without much vertical terrain that stacks contours on top of each other.

    Much of this manual work can be done in Global Mapper. Then GM can generate an elevation grid. Once you review the grid, you may want to spend some time adding additional points to help the program define ridges and drainages, as well as other rapid changes in slope.

    It will be a project any way you do it.

    Interesting links that you probably know about, and I don't see any pricing other than free:
    NEDF Portal
    Geophysical Archive Data Delivery System

    Otherwise 1-sec SRTM data seems to be the more common reference. Their intents must be for wide-area projects. But LIDAR pops up here and there ...

    Tucson, AZ
    (office in Adelaide, SA)
  • PhilRatcliffePhilRatcliffe Global Mapper User Posts: 27Trusted User
    edited January 2015
    Thank you Mykle, gives me some research to do Cheers, Phil
    (office in Wodonga, Vic) :)
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